The present invention relates to a fuel injection control apparatus for an alcohol engine capable of setting a fuel injection timing during high load running at a low speed in accordance with an alcohol concentration of fuel.
Alcohol engines for flexible fuel vehicles (FFV) have been developed recently which can use gasoline only, alcohol only or a mixed fuel of gasoline and alcohol.
The alcohol concentration (content) of the fuel used for an alcohol engine changes from 0% (only gasoline) to 100% (only alcohol) depending upon the circumstance when a user supplies the fuel. Since the theoretical air/fuel ratio changes with the alcohol concentration of the fuel, it is necessary to change an air/fuel ratio and an ignition timing properly in accordance with the alcohol concentration of the fuel, as described for example in Japanese Patent Laid-open Publication No. 56-66424.
Conventionally, the fuel injection timing has been set generally such that the fuel is injected within the intake pipe and that the fuel injection is terminated before the start of air intake, so as to enhance carburetion and stabilize combustion.
The cubic expansion coefficient of alcohol for the evaporation thereof is about three times as large as that of gasoline, so that the volumetric efficiency is lowered more by fuel evaporation as the alcohol concentration of the fuel becomes higher.
The fuel injection amount is determined in accordance with the above-described conventional technique irrespective of the alcohol concentration. The higher the alcohol concentration, the more the volumetric efficiency decreases due to carburetion. The phenomena results in an inability of proper air/fuel control, lower running performance and poor exhaust emission.
Particularly in the case of the low speed, high load running range where a fuel injection amount is large and the injected fuel remains within the intake pipe for the period sufficient for evaporation of the fuel, the volumetric efficiency is degraded to a great extent, and the above-stated problems become significant.
In order to maintain an air/fuel ratio of an alcohol engine at a theoretical air/fuel ratio, as disclosed in Japanese Patent Laid-open Publication No. 57-76231, it becomes necessary to increase the fuel injection amount as the alcohol concentration of the fuel increases so that the usual injector used by the general fuel injection system becomes inadequate in its capacity.
Specifically, the theoretical air/fuel ratio for a fuel of 100% alcohol is generally about one half that for a fuel of 100% gasoline. At the same running condition, it is therefore necessary to make the injector capacity about two times as large. Accordingly, a specific injector is required to ensure a requested fuel injection amount, resulting in an increase of cost.